1. Field of the Invention
The present invention relates to a semiconductor device using an SOI (silicon on insulator) substrate and a manufacturing method of the semiconductor device. In particular, the present invention relates to a bonding SOI technique, a semiconductor device using an SOI substrate which is obtained in such a manner that a single crystal semiconductor film or a polycrystalline semiconductor film is bonded to a substrate with an insulating film interposed therebetween, and a manufacturing method of the semiconductor device.
2. Description of the Related Art
Demands for higher integration, higher operation speed, higher performance, and lower power consumption have been more stringent, and in response to the above-described demands, a transistor using an SOI substrate has attracted an attention as effective means that can take the place of a bulk transistor. Higher operation speed and lower power consumption can be expected more in the transistor using the SOI substrate than in the bulk transistor because an insulating film is formed over a semiconductor film in the transistor using the SOI substrate, and accordingly parasitic capacitance can be reduced and generation of leakage current flowing through the substrate can be suppressed. In addition, since the semiconductor film used as an active layer can be thinned, a short channel effect can be suppressed; thus, an element can be miniaturized, and higher integration of a semiconductor integrated circuit can be realized accordingly. Moreover, since the transistor using the SOI substrate is completely latch-up free, there is no possibility that the element is broken due to heat caused by latch-up. Furthermore, the transistor using the SOI substrate does not need element separation unlike the bulk transistor; thus, the transistor using the SOI substrate has advantages in that the distance between the elements can be shortened and higher integration can be realized.
As one of manufacturing methods of an SOI substrate, there is a method in which a semiconductor film is bonded to a substrate with an insulating film interposed therebetween, such as UNIBOND (registered trademark) typified by Smart Cut (registered trademark), ELTRAN (epitaxial layer transfer), a dielectric separation method, or a PACE (plasma assisted chemical etching) method. The above-described bonding methods make it possible to form a high-performance integrated circuit, in which a single crystal semiconductor film is used, over all kinds of substrates.
However, in the transistor using the SOI substrate, junction capacitance is formed between an impurity region of the semiconductor film and the substrate. Reduction in the junction capacitance is a very important object for realizing further reduction in power consumption of a semiconductor integrated circuit. It is desired that a material having low permittivity while keeping its insulating property be used for the insulating film interposed between the impurity region and the substrate.
In Reference 1 (Japanese Published Patent Application No. 2001-144276), a transistor having an SON (silicon on nothing) structure in which a cavity is provided between a semiconductor film and a substrate is described. In addition, in Reference 2 (Japanese Published Patent Application No. 2004-146461), a structure of an SOI substrate in which a cavity is provided between a semiconductor film and a substrate is described. The substrate and the semiconductor film are insulated from each other with the use of air, relative permittivity of which is 1, whereby junction capacitance can be reduced.